A programmable logic device, sometimes referred to as programmable application specific integrated circuit (PASIC), field programmable gate array (FPGA), or complex programmable logic device (CPLD), is a versatile integrated circuit chip, the internal circuitry of which may be configured by an individual user to realize a user-specific circuit design. To configure a programmable logic device, the user configures an on-chip interconnect structure of the programmable logic device so that selected input terminals and selected output terminals of selected on-chip circuit components are electrically connected together in such a way that the resulting circuit is the specific circuit design desired by the user.
The architecture of programmable logic devices, which includes logic cells and a routing network, is such that many different circuit designs may be implemented by appropriately programming the programmable logic device. Moreover, unlike application specific integrated circuits (ASICs), there is no one specific circuit design that is to be implemented with the programmable logic device. Thus, while ASIC devices can accommodate specific power conservation designs using different and/or multiple custom sized power islands, it is not possible to fabricate a generic programmable logic device with custom sized circuit specific power conservation designs.